Zirconia is a mineral with a crystal structure made from the elements zirconium and oxygen. It looks pretty like diamond, but is only worth a fraction of the value.
Sapphires and rubies are both crystals of the mineral corundum - but with different impurities to create blue and red hues. Australian sapphires are renowned for being inky blue.
What makes a stone a gem? It boils down to a few key qualities: beauty and durability. But opal, the national gemstone of Australia, is an anomaly - it's soft.